The Cell Physiology Core supports investigators by providing access to video-enhanced microscopy and conventional transepithelial electrophysiology. The Core maintains these facilities and trains investigators in their use. The video microscopy set-up includes an upright microscope with epifluorescence and DIC and phase optics. The images can be captured simultaneously by a conventional, high resolution video camera (transmission image) and an intensified CCD camera (for fluorescence), continuously digitized and analyzed on-line by fluorescence software package. A filter wheel can change the wavelength of excitation for fluorescence, enabling intracellular Ca or pH measurements by fluorescent probes and the ratio method. A photonic mode is particularly well suited for the analysis by low-fluorescence immunocytochemistry. This core will be extensively used for cytosolic Cl, Na, K, and pH measurements by Projects #1, #3, and #4, and also for immunofluorescence by Projects #2, #4, and 5. Projects #3 and 5 will also take advantage of the capability to measure short-circuit current, potential, and conductance of confluent monolayers grown on porous filter supports.